The present invention relates to a dielectric resonator in which a strip line is formed three-dimensionally on a dielectric substrate, and a dielectric filter using such a dielectric resonator.
A filter for microwaves is known which uses a resonator consisting of a strip line. For example, a 1/4 wavelength resonator is formed such that one end of a linear strip line type resonator conductor provided on a dielectric substrate is set free and the other end thereof is short-circuited to an earth electrode. The resonator conductor is set to a length which is an odd number of times as large as 1/4 of the resonance wavelength. In order to actually form a filter, such resonator conductors are arranged on a dielectric substrate, and the short-circuited ends thereof are connected together, the distance between adjacent resonator conductors being so determined that the degree of coupling corresponds to the filter characteristics.
In the field of 1/2 wavelength resonators a technique for forming a resonator conductor may be provided on a dielectric substrate in the shape of a hairpin as disclosed in Japanese Patent Publication (Unexamined) Nos. 62-193302/1987 and 58-103202/1983. By the technique, an actual filter is so formed that a plurality of hairpin-shaped resonator conductors are parallel-connected together.
In any case, conventional strip line type filters are such that predetermined resonator conductors are provided on a dielectric substrate. Namely, the resonator conductors are arranged in a planar configuration. For example, conductive paste is laid in a predetermined shape on a dielectric substrate by screen printing, or in a dielectric substrate in a recess of predetermined shape where an electrode is formed or is made. Then, the interior of the recess is filled with conductive paste. The dielectric substrate is then heated to bond the conductive paste thereto by firing and form a desired electrode pattern. There is another example having a triplate structure in which a thin conductive plate (resonator conductor), molded into a predetermined shape in advance by punching or etching, is sandwiched between two dielectric substrates as shown in Japanese Patent Publication (Unexamined) No. 57-204602/1982.
In any of these conventional strip line type filters, resonator conductors are arranged, in a plane, on a dielectric substrate, so that the areas of the filters cannot be reduced. Especially, in the case where resonator conductors are connected in a plurality of steps to form a filter, the horizontal size of the filter necessarily increases, even though the height thereof is unchanged.
In the resonator pattern obtained by screen printing, the printing accuracy is generally low, and the frequency and degree of coupling vary, so that it becomes necessary to subject such a resonator pattern to trimming. In the case where a recess formed in a dielectric substrate is filled with conductive material, the conductive material unavoidably adheres extensively to even an unnecessary portion of the substrate in most cases. Consequently, such a conductive material has to be removed, i.e., a troublesome additional operation is required. In the case of a triplate structure, a clearance, the height of which corresponds to the thickness of a conductive plate, is produced between two dielectric substrates due to the presence of the conductive plate. Therefore, the equivalent dielectric constant lowers, and the dimensions of the filter cannot be reduced. Moreover, it is necessary to provide a suitable shielding means for the thickness portions of the conductive plate.